Frequency-modulated radio altimeter



Oct. 7, 1941. l. woLFF ET AL 2,257,830

FREQUENCY-MODULATED RADIO ALTIMETER Filed Sepb. l, 1939 msnm Ww ,www n0W fwfm/a/vcy Y mwpuE/vcr RECH/v51? y .Y /WF'L/f'f'f V METER L 1 (a .5.9 V 7 lMiner/msmW211i' a e dc /mafcr Wma? MPL/PIER 1FL/F151? me Pan/EHsol/RC5 fw? Maron z5 'HL 7'/ TUBE Ctttorneg Patented Oct. 7, 1941vnur:QUENor-Montanara Ramo ALTIMETER Irving Wolff, Merchantville, andRoyden C. Sanders, Jr., Audubon, N. J., assignors to Radio Corporationoi'vAmerica, a corporation of Delaware Application September 1, 1939,Serial No. 293,128

14 Claims.

ing the facts '(1) that the maximum receiver band width is required atthe maximum altitude-the point of weakest signals-and (2) that one lofthe limiting factors of ampliiication is the noise arising within theamplifier. These two factors bear a relation which is also well known i.e., the noise voltage in the output of an amplifier is proportional tothe square root of the amplified frequency band and directlyproportional to the amplication.r

In other'words, if,.the frequency band being amplified were reduceditol/nth of its origina .l

value, the amplification could be increased \/n times the original valueand the original noise level would be unaltered. While these factors maynot be of outstanding importance for short ranges or low altitudes, theyare of great importance at the long ranges or high altitudes. especiallywith a low power transmitter.

It is one ofthe objects of the invention to provvide afrequency-modulation distance measuring device of increased range anddecreased band width. Another object is to provide means in a radioaltimeter for making the modulation band approximately inverselyproportional to the altitude. An additional object is to provide meansfor improving the ratio of signal to amplier noise by decreasing themodulation band width as the signal strengthdiminishes.

The invention will be described by referring to the accompanying drawingin which Figure 1 is a schematic diagram of one embodiment of theinvention; and Figure 2 is a graph illustrating the operation of theinvention.

y Referring to Fig. 1, a radio receiver I is connected through a lowfrequency amplier 3 to a frequency meter 5 of the electronic type. Oneform of frequency meter is shown in the copending application Serial No.248,577, filed December 30, 1938, by Royden C..Sanders, Jr., andentitled Frequency meter.

The frequency meter output is connected to a D. C. amplier 1, whichcontrols the gain of an A. C. amplier 9, in the manner of the well knownautomatic volume control connections which supply automatically biasingpotentials to the amplier to be controlled. In the present arrangementthe A. V. C. control opposes the normal amplifier bias, as is also knownin the art. The input of the A. C. amplier is connected to an A. C.power source and the amplifier output is applied to the motor 25 ofafrequency modulator I3 which is connected to a radio transmitter I5. AnA. C. voltmeter Il, calibrated as an altimeter, is connected to thepower leads to the motor 25.

All of the foregoing components, with the possible exception of thefrequency modulator I3, are well known to those skilled in the art and,therefore, do not require a detailed description. The frequencymodulator may be of the variable capacitor type driven by a plunger typemotor 25. A pair of xedly mounted capacitor plates I9, 2| is secured toa frame 23 on which is mounted an electromechanical motor 25. Themovable element of the motor moves back and forth as indicated by thearrow 2l. A movable capacitor plate 29 is fastened to the movableelement. As the movable plate 29 is vibrated back and forth by currentsapplied to the motor from the low frequency oscillator II through thelow frequency amplifier 9, the capacity of the fixed plates I9, 2| isvaried. While the rate of variation of the capacitor is determined bythe applied frequency, the variation in capacity is directlyproportional to the applied current, and inversely proportional to theeffective dielectric between the capacitor plates. This capacityvariation is applied to the transmitter by connecting the capacitor I9,2l across the resonant circuit of the transmitter to vary itsinstantaneous frequency. It should be understood the lnductive variationmay be used and such variation may be applied by means of a motor orelectronically.

If an alternating current of a constant peak amplitude were applied tothe motor 25, the movable plate 29 attached to the motor shaft wouldvibrate at the frequency of the applied current and between limitsdetermined by the amplitude of the current. Such movement would resultin a variation of capacity of the plates I9, 2| at a xed rate andbetween fixed limits. Since the capacity between the'plates I9, 2| ispart of the resonant circuit of the transmitter I5, a frequencyvariation or modulation of constant band width would be produced. Thiswould be the case if the D. C. ampliller 1 and connections thereto wereomitted from the circuit of Fig. 1.

However, the D. C. amplifier 1 is included in the circuit and issupplied with D. C. from the frequency meter from which unidirectionalor rectified currents are obtained. These currents are amplified by theD. C. amplifier I and are applied to the low frequency amplifier 9 sothat the gain of the latter amplifier 9 is reduced. The reduced gain ofthe low frequency amplifier reduces the amplitude of the currentssupplied from the low frequency oscillator through the amplifier 9, tothe motor 25 (see Fig. 3) of the frequency modulator I3. Since currentsof lesser amplitude are applied to the motor, the pistonllke excursions21 of the motor shaft will be reduced. This in turn reduces themovements of the plate 29 and hence reduces the capacity variation ofthe fixed plates I9, 2| from what would be obtained from the applicationof currents of larger amplitude. In turn,. the frequency modulation ofthe transmitter covers a narrower band width because the fixed platesare connected to the resonant circuit of the transmitter, as previouslymentioned. The relation between the frequency modulation band of thetransmitter and the altitude or distance from which measurements are tobe made will be hereinafter described.

The several factors determining the applied currents are indicated bythe formula Received heterodyne frequency:

(Mod. freq.) (Altitude) (Band width) where the heterodyne and modulationfrequencies are expressed in cycles per second, the altitude in feet,the band width in megacycles per second, and K=250. From the formula,which has been determined mathematically and ex' perimentally, itfollows that for a given modulation frequency and altitude range theamplifier band width is proportional to the frequency modulation bandwidth. However, the transmitted band width must be large enough toobtain the desired accuracy at the lowest height to be indicated. Whilethe lowest modulation frequency should not be less than 15 cycles persecond, 120 cycles per second is preferred.

In the prior art radio altimeters, the usual procedure has been toadjust the modulation band width for the desired accuracy at the lowaltitudes and to maintain this band width for all altitudes and toaccept the aforementioned disadvantages. In the altimeter of the presentinvention, the method is to adjust the modulation band width just wideenough to obtain the desired accuracy at the lower altitudes asindicated by the flat portion 3| of the graph 33 of Fig. 2. The initialmodulation band width is determined by the initial space between thefixed plates I9 and 2| and the movable plate 29 and the currentinitially applied to the motor 25. Thereafter, the received waves areconverted into a D. C. bias which overcomes the normal bias of the A. C.amplifier 9 and thus diminishes the amplier gain. The diminished gainreduces the amount of current applied to the motor of the frequencymodulator I3 and hence the modulation band is diminished as indicated bythe curved portion 35 of the graph 33. The resultant effect on theheterodyne frequency is indicated by the graph 31. The altitude ismeasured in terms of the A. C. lvoltage (or current) applied to themotor 25 .of the frequency modulator. erable to calibrate the A. C.voltmeter |1 tol indicate directly the altitude in feet or other desiredunits. It should be understood that the voltage or current correspondingto the altitude may be measured at other points in the system. Forexample, the controlling bias or voltage may be measured to indicate thealtitude.

Thus the invention has been described as a radio altimeter of thefrequency modulation type in which the modulation band width is variedin the conventional manner to obtain the desired accuracy at lowaltitude. The modulation band width is then made smaller and smaller asthe altitude is increased. The control of the band Width is broughtabout by applying a direct current bias which is derived from thereceived waves. The diminution of the modulation band permits increasedamplification in the receiver because the signal-to-noise ratio isproportional to the square root of the frequency band amplified in thereceiver. The receiver, transmitter, amplifiers, oscillators andmodulators used in the practical application of the invention may be anyof the several well known types.

We claim as our invention:

1. A radio altimeter including means for transmitting radio waves ofvarying frequency to be reflected from the earth, a receiver forreceiving said signals and for producing a signal corresponding infrequency to the difference in frequency of the transmitted andreflected waves, means connected to said receiver for deriving from saidreceiver currents h aving an amplitude proportional to the differencefrequency of the received signals, a low frequency oscillator, afrequency modulator including connections to said transmission means, anamplifier connecting said oscillator and modulator, and means connectingsaid current deriving means and said amplifier for applying said derivedcurrents to said amplifier to control its gain.

2. A radio altimeter including means for transmitting radio waves ofvarying frequency toward the earth to be reflected therefrom, a receiverfor receiving said transmitted waves directly and after reflection,meansA connected to said receiver for deriving from said receivercurrents having an amplitude proportional to the dierence in frequencyof the directly received waves and the received' reflected waves, a lowfrequency oscillator, a frequency modulator connected to said means fortransmitting, an amplifier connecting said oscillator and modulatorforapplying currents from the former to the latter, means connecting saidcurrent deriving means and said amplifier for applying said derivedcurrents to said amplifier to control its gain, and means for indicatingaltitude as a function of the current applied by said amplifier to saidfrequency modulator. l

3. A radio altimeter including a transmitter for radiating radio wavesof varying frequency toward the earth to be reflected therefrom, areceiver for receiving said transmitted waves directly and afterreflection, means connected to said receiver for deriving from saidreceiver currents having an amplitude proportional to thel to applycurrents from the oscillator to the mod` ulator, means connected to saidcurrent deriving means and to said amplifier for applying said -derlveacurrents to said amplifier to control its gain, and means connectingsaid frequency modulator to said transmitter' whereby the modulationfrequency is determined by said low frequency oscillator and the modation band width is controlled as a fimction of the derived currents.

4. A radio altimeter including a transmitter for radiating altitudedetermining radio waves of varying frequency toward the earth to bereected therefrom', a receiver directly responsive to said transmittedwaves and to said reflected waves, means connected to said receiver forderiving from said receiver currents having an amplitude proportional tothe frequency difference of the waves received directly and afterreiiection, a low frequency oscillator, a frequency modulator, anamplifier connecting said oscillator and modulator for applying currentsfrom the oscillator to the modulator, means connected to said currentderiving means and said amplifier for applying said derived currents tosaid amplier to control its gain, means connecting said frequencymodulator to said transmitter to vary its frequency whereby themodulation frequency is a function of said low frequency oscillatorfrequency and the modulation band Width is a function of the amplitudeof said derived currents, and a meter for indicating altitude as afunction of the currents applied by said amplifier to said frequencymodulator.

5. A radio distance-measuring device of the frequency modulator typeincluding means forI transmitting waves to an object to be Areflected\therefrom, means for receiving said waves di-y rectly from saidtransmitting means and for receiving said waves after reflection, meansconnected to said transmitting means for varying the frequency of thetransmitted waves, means connected to said receiving means for derivingcurrents having an amplitude proportional to the difference in frequencyof the waves directly received and the waves received after reflection,and means connecting said current deriving means and said frequencyvarying means for applying currents corresponding to said derivedcurrents to control the frequency limits of the frequency varying means.

6. A radio distance-measuring device of the frequency modulator typeincluding means for transmitting radio waves toward an object to bereflected therefrom, means for receiving said transmitted waves directlyand after refiection, means connected to said transmitting means forvarying the frequency of the transmitted waves,

means connected to said receiving means for deriving currents having anamplitude proportional to the difference in frequency of the directlyreceived waves and the waves received after reflection, means connectingsaid current deriving means and said frequency varying means forapplying currents corresponding to said derived currents to control theband width of the transmitted waves, and means for indicating distanceof said obiect as a function of said band width.

'7. A radio distance-measuring device of the frequency modulator typeincluding means for transmitting frequency varying radio waves toward anobject to be reected therefrom, means for receiving directly saidtransmitted waves and for receiving said waves after reflection, meansconnected to said transmitting means for varying the frequency of thetransmitted waves, means connected to said receiving means for derivingcurrentshaving an amplitude proportional to the beats of' the directlyreceived transmitted waves and the received reflected waves.

and means connecting said current deriving.

means and said frequency varying means for applying currentscorresponding to said derived currents to control the band width of thetransmitted waves.

8. A radio distance-measuring device of the frequency modulator typeincluding means for ing the frequency of the transmitted waves..

means connected to said receiving means for deriving currents having anamplitude proportional to the beats of'the directly received transmittedwaves and the received reflected waves, means connecting said currentderiving means and said frequency varying means for applying currentscorresponding to said derived currents to control the amount of thefrequency variation of the transmitter, and means connected to saidfrequency controlling means for indicating distance as a function of theamount of frequency variation of the transmitting means.

9. A radio distance-measuring device of the frequency modulator typeincluding means for transmitting frequency varying waves toward anobject to be reflected therefrom, a receiver responsive to thetransmitted and reflected waves, means connected to said transmittingmeans for varying the frequency of the transmitted waves, meansconnected to said receiving means for deriving currents having anamplitude proportional to the beat of the waves received directly andafter reflection, means connecting said current deriving means and saidfrequency varying means for applying currents corresponding to saidderived currents to control the band width of the transmission, andmeans connected to said band width control means for delaying theapplication of the controlling currents for the measurement of distancesless than a predetermined length.

10. A radio distance-measuring device of the frequency modulator typeincluding means for transmitting waves toward an object to bereflectedtherefrom, means for receiving said transmitted waves directly and afterreflection, means connected to said transmitting means for varying thefrequency of the transmitted waves, means connected to said receivingmeans for deriving currents having an amplitude proportional to thedifference in frequency of the waves received directly and afterrefiection, means connecting said current deriving means and saidfrequency varying means for applying to the latter currentscorresponding to said derived currents to control the band width of thetransmission, means eifectively connected to said frequency controllingmeans for delaying the application of the controlling currents for themeasurement of`distances less than a predetermined length, and means forindicating distance as a function of the band width of the transmitter.

1l. The method of measuring distance which includes transmitting a wave,varying the* fre- 12. The method of measuring distance which includestransmitting a wave, varying the frequency of said wave. receiving saidwave directly and after reection, heterodyning the direct and reflectedwaves. deriving a voltage corresponding to the difference of frequencyof said direct and reflected waves, applying said voltage to control thefrequency limits of said frequency variation as a function of saiddistance, and measuring the voltage applied to the frequency control toindicate distance.

13. The method of measuring distance which includes transmitting a wave,varying the frequency of said wave, receiving said wave directly andafter reflection, heterodyning the direct and reflected waves, derivinga voltage corresponding to the difference in frequency of said directand reflected waves, generating a low frequency current. amplifying saidlow frequency current, controlling said amplification by said derivedvoltage, and applying said controlled amplified voltage to determine thefrequency sweep of said frequency variation.

14. 'I'he method of measuring distance which includes transmitting awave, varying the frequency of said wave, receiving said wave directlyand after reflection, heterodyning the direct and reflected waves,deriving a voltage corresponding to the difference in frequency of saiddirect and reflected waves, generating a low frequency current,amplifying said low frequency current, controlling said ampliflcation bysaid derived voltage, applying said controlled amplified voltage todetermine the frequency sweep of said frequency variation, and measuringthe controlled amplined voltage to indicate distance.

IRVING WOLF'F. VROYDEN C. SANDERS. Jl.

